Log4j FileAppender recreating deleted files

Question

I am using Log4j as logging framework in a project I am working on. I have the following situation: Log4j is configured to write the logs into a log file. At some point, this log file is copied to another destination and deleted. Logging framework keeps working, but the logs are not written to the log file because it is deleted. Is there any way to tell Log4j to recreate the file and keep writing the logs into the log file.

Best regards, Rashid

Answer 1

I study the source of log4j and find log4j can't create new log file, it just print the error message to system.err when the log file was deleted

    /** 
     This method determines if there is a sense in attempting to append. 

     <p>It checks whether there is a set output target and also if 
     there is a set layout. If these checks fail, then the boolean 
     value <code>false</code> is returned. */  

  protected   boolean checkEntryConditions() {  
    if(this.closed) {  
      LogLog.warn("Not allowed to write to a closed appender.");  
      return false;  
    }  

    if(this.qw == null) {  
      errorHandler.error("No output stream or file set for the appender named ["+  
            name+"].");  
      return false;  
    }  

    if(this.layout == null) {  
      errorHandler.error("No layout set for the appender named ["+ name+"].");  
      return false;  
    }  
    return true;  
  }  

I think there are two workaround

1. create another cron thread to monitor the log file
2. add judge in getLog or getInstance (singleton), check the log file does exist, if not then init log4j

Answer 2

Make sure you can declare this line in your log4j file

    log4j.appender.rollingFile.File=D:/myapp/mylog.log

If you already declared it, you log file can delete or replace as you like. Then you rerun your program and new log file is created in this path.

Answer 3

Try This Class

package wodong.test;
import java.io.File;
import java.io.IOException;

import org.apache.log4j.FileAppender;
import org.apache.log4j.spi.LoggingEvent;

public class LastFileAppender extends FileAppender {
    @Override
    public void append(LoggingEvent event) {
        checkLogFileExist();
        super.append(event);
    }
    private void checkLogFileExist(){
        File logFile = new File(super.fileName);
        if (!logFile.exists()) {
            try {
                logFile.createNewFile();
            } catch (IOException e) {
                System.out.println("Error while create new log file.");
            }
        }
    }
}

Also edit the log4j config file

log4j.appender.R=wodong.test.LastFileAppender

Answer 4

I went through the source code of log4j. When a FileAppender/RollingFileAppender is initialized, a FileOutputStream instance is created pointing to the File. A new FileDescriptor object is created to represent this file connection. This is the reason, the other solutions like Monitoring the file through Cron and Creating the File in append method by overriding didn't work for me, because a new file descriptor is assigned to the new file. Log4j Writer still points to the old FileDescriptor.

The solution was to check if the file is present and if not call the activeOptions method present in FileAppender Class.

package org.apache.log4j;

import java.io.File;
import org.apache.log4j.spi.LoggingEvent;

public class ModifiedRollingFileAppender extends RollingFileAppender {

    @Override 
    public void append(LoggingEvent event) {
        checkLogFileExist();
        super.append(event);
    }

    private void checkLogFileExist(){
        File logFile = new File(super.fileName);
        if (!logFile.exists()) {
            this.activateOptions();
        }
    }
}

Finally add this to the log4j.properties file:

log4j.rootLogger=DEBUG, A1
log4j.appender.A1=org.apache.log4j.ModifiedRollingFileAppender
log4j.appender.A1.File=/path/to/file
log4j.appender.A1.layout=org.apache.log4j.PatternLayout
log4j.appender.A1.layout.ConversionPattern=%d{yy/MM/dd HH:mm:ss,SSS} %p %c{1}: %m%n

//Skip the below lines for FileAppender
log4j.appender.A1.MaxFileSize=10MB
log4j.appender.A1.MaxBackupIndex=2

Note: I have tested this for log4j 1.2.17

Answer 5

Try This one. I do not have a Linux machine right now, so I'm not sure if it can resolve the performance issue.

package wodong.test;
import java.io.File;
import java.io.IOException;

import org.apache.log4j.FileAppender;
import org.apache.log4j.spi.LoggingEvent;

public class LastFileAppender extends FileAppender {
    @Override
    public void append(LoggingEvent event) {

        checkLogFileExist();
        super.append(event);
    }

    private void checkLogFileExist() {
        if (qw == null) {
            File logFile = new File(super.fileName);
            if (!logFile.exists()) {
                try {
                    logFile.createNewFile();
                } catch (IOException e) {
                    System.out.println("Error while create new log file.");
                }
            }
        }
    }
}